This invention relates to a valve mechanism for an internal combustion engine and, more particularly, to an improved cylinder head and valve actuating system for an overhead camshaft internal combustion engine.
It is well recognized that the performance of internal combustion engines can be improved through the use of overhead valves operated by overhead mounted camshafts. When an overhead camshaft arrangement is employed, the cylinder head assembly tends to become much more complicated than with a conventional pushrod operated engine. This is because the cylinder head must, in addition to supporting the valves and the valve springs, provide support for the camshaft and for the actuators for the valves. Frequently, the valves are directly operated and this means that an arrangement must be provided for slidably supporting the thimble tappets that operate the individual valves from the camshaft.
If all of these functions are performed primarily by the cylinder head, then the cylinder head casting becomes extremely complicated. In addition, since it is necessary to machine the bearings for the camshaft provided by the cylinder head, and the bores for slidably supporting the tappets, then machining operations also add significantly to the cost of the cylinder head. Furthermore, there is the problem of assembly of all of the components into such a unitary cylinder head assembly, and the problems of accessibility of the various fasteners for securing the cylinder head to the cylinder block and the bearing caps to the cylinder head for journalling the camshaft. Of course, the problems mentioned above are complicated when the engine employs twin overhead camshafts.
It has been proposed, therefore, to employ a construction wherein the main cylinder head member itself does not have to perform all of these functions. For example, in U.S. Pat. No. 4,612,885, entitled Camshaft Bearing Arrangement For Overhead Cam Engine, issued Sep. 23, 1986, in the name of Masaaki Yoshikawa, and assigned to the assignee hereof, there is depicted a cylinder head arrangement wherein the main cylinder head assembly only supports the poppet valves for their movement and the return springs for the poppet valves. The camshafts and valve actuating tappets are supported in a separate cam carrier that is affixed to the cylinder head and thus can be machined and cast separately simplifying the aforenoted problems. However, with the arrangement shown in that Patent, the cam carrier forms the outer periphery of the cylinder head and the cam cover must sealingly engage it. In addition, the cam carrier must have a sealing arrangement around its outer periphery with the upper surface of the cylinder head to afford sealing. Hence, substantial additional machining operations are required.
It is, therefore,a principle object of this invention to provide an improved cylinder head assembly for an overhead camshaft internal combustion engine.
It is a further object of this invention to provide an improved cylinder head assembly for an overhead cam internal combustion engine wherein at least some of the valve actuating tappets and at least one of the camshafts are supported by a separate cam carrier member that is affixed to the cylinder head but in such a way that this cam carrier member need perform no sealing functions for the overall cylinder head assembly.
It is a further object of this invention to provide an improved and simplified cylinder head assembly for an internal combustion engine having an overhead camshaft wherein machining and assembly operations are considerably simplified.
In addition to the problems already noted, it is also desirable to ensure that the cam actuating tappets and camshafts are well lubricated. When a separate cam carrier member is provided, this can present additional difficulties in insuring that the camshafts are adequately lubricated as are the valve actuating tappets.
It is, therefore, a still further object of this invention to provide an improved cylinder head assembly including an improved arrangement for lubricating the camshaft journals thereof.